Gav'Et-Oni Achievement in Vampire Survivors

Kappa are a new enemy in the new map only. They spawn only in two known locations: directly south by the Gold Ring on the map, and in the northwest corner of the map where a frozen lake is. From my experience, they spawn very slowly in the south, so definitely go to the northwest corner. To get there, from the spawn, head up and then take the left path, eventually going up, through a cave, out the left of it and left through a very narrow path, and that eventually opens up into the frozen lake area. There will be a giant skeleton enemy in the middle, but if you stay on the lake Kappa enemies will spawn nonstop around you the entire time. They unfortunately don't swarm in huge numbers, so if possible, I recommend getting some curse to speed up the process.

You unfortunately can't tell during the run whether you've killed enough, but make a note of your kill count when you get to the lake and then kill an additional 10,000 enemies or so. That should be plenty to reach 6000 Kappa and unlock this achievement.